Rolling,Microstructure And Mechanical Properties Of Ti-44Al-8Nb-0.2W-0.2B-Y Alloy Sheet

The high Nb containing TiAl alloys have the advantages of high specific strength,excellent high temperature oxidation resistance as well as creep resistance and the high Nb containing TiAl alloy sheets can expand the application of TiAl alloy.However,the intrinsic brittleness,poor hot workability,narrow hot working window,large deformation resistance and coarse lamellar colony,etc.severely increase the difficulty to fabricate high Nb containing TiAl alloy sheet s.In this paper,the as-forged Ti-44Al-8Nb-0.2W-0.2B-Y alloy was investigated to study the effect of rolling parameter on formability,microstructure and tensile properties of TiAl alloy sheets.Moreover,the deformation behavior of the high temperature disorder ? phase was studied and the microstructure evolution model during rolling at different temperatures were established.The as-forged alloy has the nearly lamellar microstructure with average lamellar colony size of 120 ?m and the ? grains with mean grain size of 10 ?m distributed around lamellar colonies.Reducing strain can expand the hot working window of high Nb containing TiAl alloy to relatively low temperature and high strain rate region.When deformation strain decreases from 50% to 25 %,the ensured sample forming by isothermal compression conditions can be expanded from 1150 ?/0.1 s-1 to 1050 ?/0.5 s-1 with power dissipation rate reduction under the same deformation conditions.The step-by-step hot compression of TiAl alloy shows that the workability of TiAl alloy is improved after one hot compression deformation due to the increased volume fraction of equiaxed ? grains and ? grains and the decreased volume fraction and size of lamellar colony.Based on thermal processing map and microstructure optimization,the rolling process with strain rate lower than 0.5 s-1,deformation strain less than 25% and deformation temperature higher than 1150 ? was determined.The misorientation in ? lamella is further increased parallel to the length of ? lamella when deformed above Te temperature,resulting in the rotation degree of ? lamella along length direction is higher than that along thickness direction,which leads to form some subgrains.Moreover,these subgrains continue to rotate during deformation to increase misorientation and eventually some new ? grains are formed along the length of ? lamella.Furthermore,the ? groove-like grain formed on ? lamella also accelerates the decomposition of ? lamella.When ? grain goes through ? lamella,the parent ? lamella will be completely separated,and ultimately be transformed into non-continuous equiaxed ? grains.It is beneficial for the conversion of ? lamella into ? grains when using lower strain rate and larger deformation.After three time's isothermal compression deformation under the conditions of 1250 ? / 0.1 s-1 / 25%,the coarse lamellar colonies were completely transformed into equiaxed ?2 grains and ? grains.Yet,the lamellar colonies are not be completely decomposed due to temperature drop and high rolling rate in actual rolling.With the increas of deformation temperature,the content of disorder ? phase and the proportion of equiaxed ? grains increases above Te temperature,and the deformability of TiAl alloy increases.Moreover,with the rolling continues,the equiaxed ? grains and ? grains increase,and continuous dynamic recrystallization and discontinuous dynamic recrystallization occur in ? grains and ? grains,respectively,forming finer ? grains and ? grains.Furthermore,these recrystallized ? grains will undergo ? ? ? phase transition during rolling with temperature drop and turn into ? grains and finer lamellar colonies.The degree occurrence of the above mentioned two phase transformation at different temperatures is different.Therefore,TiAl alloy sheets with near ? microstructure,duplex microstructure and nearly lamellar microstructure can be fabricated directly by controlling rolling temperature.The high Nb containing TiAl alloy sheets with total deformation of 58.4% and 72.8% can be easily fabricated using 20% rolling reduction per pass due to the higher than 95 vol.% equiaxed ? grain at 1290? and 1305 ?,with good deformation capacity.The finer ? recrystallized grains can be firstly formed and then new lamellar colonies are formed by the precipitation of ? lamellae from recrystallized ? grains during rolling.But the mean colony size of the sheet is often relatively large such as 60 ?m and 80 ?m.The as-rolled alloy fabricated at 1305 ? has excellent tensile property and the tensile strength is up to 970 MPa at 800 ?.When rolling at 1200 ?,these equiaxed ? grains transform into ? grains,so the microstructure of sheet is near ? microstructure.When the total rolling reduction is 68%,the average grain size is only 6.8 ?m.The alloy has excellent plasticity at high temperature and it has a superplasticity of 290% when deformed at 950 ? and 2.5 × 10-4 s-1.When rolling at 1150 ?,the workability of the TiAl alloy is poor due to the large lamellar colony and a small amount of disorder ? phase(12 vol.%),resulting in the TiAl alloy sheet appears cracking when the total deformation is 56% with 15% rolling reduction per pass.The content of disorder ? phase is over 70 vol.% after annealing at 1250 ? for 2 h for current high Nb containing TiAl alloy.Multiple passes rolling promotes the continuous dynamic recrystallization of ? grains and ? lamella as well as the decomposition of ? lamella assisted by ? grains.And the recrystallized ? grains also undergo eutectoid transformation during rolling to form new lamellar colonies.Meanwhile,the ? grains and the ? lamellae are also transformed into equiaxed ? grains.The proportion and size of remnant lamellar colony can be reduced and the newly formed lamellar colony can be increased by increasing rolling reduction.As total rolling reduction increases from 43% to 72%,the volume fraction of remnant lamellar colonies decreases from 28.5% to 6.8% and newly formed ?2/? lamellar colonies increases from 13.7% to 33.7%.The increase of total deformation also contributes to the improvement of tensile properties and the adverse influence of remnant lamellar structure on tensile properties also can be reduced.When total deformation is 72%,the tensile strength and ductility of duplex microstructure is 1106 MPa and 0.94%,respectively,which is better than 1100 MPa and 0.52% of the sheet with nearly lamellar microstructure.Ti-44Al-8Nb-0.2W-0.2B-Y alloy sheet with total rolling reduction of 85% was successfully fabricated at 1250 ? by rolling redcution per pass of 25%.The as-rolled alloy has fine duplex microstructrue with an average grain size of 5.3 ?m.The fine homogeneous lamellar colonies,high density dislocations,a large number of substructures,twin boundaries and the solid solution of Nb contribute to the increase in strength.At room temperature,the as-rolled alloy exhibits ultimate tensile strength and ductility of 1174 MPa and 1.01%,respectively.The yield strength and tensile strength at 800 ? also remains higher than 681 MPa and 777 MPa.